Hydroxide oxidation and peroxide formation at embedded binuclear transition metal sites; TM = Cr, Mn, Fe, Co

Abstract

Key steps in electro-catalytic water oxidation on binuclear Transition Metal (TM) sites are addressed. These comprise (a) two one-electron oxidation steps of TM-OH moieties to form the corresponding two TM=O oxy-groups, and (b) a chemical step whereby the two oxy-species form a TM-O-O-TM peroxy-bridge. A test rig representing a generic low crystal field oxide support is described and employed. The energetics for homo-nuclear Cr(III-V), Mn(III-V), Fe(II-IV) and Co(II-IV) sites are compared. The uniqueness of the tyrosine/tyrosyl-radical (TyrOH/TyrO˙) reference potential for driving the oxidation steps is demonstrated. The oxidation of adsorbed TM-OH moieties on binuclear Mn and Co candidates requires an overpotential of approximately 0.5 V relative to the chosen reference potential. Correspondingly, the subsequent O-O bond formation becomes strongly exothermic, of the order of 1 eV. The hydroxide oxidation steps on binuclear CrCr and FeFe systems are, in total, exothermic by 1.21 and 0.61 eV, respectively, relative to the TyrOH/TyrO˙ reference potential. Consequently, the chemical step for transforming the TM=O moieties to the peroxo species is found to be endothermic by the order of 0.7 eV. Based on these findings, a catalyst containing one TM from each class is suggested. The validity of this concept is demonstrated for the FeCo binuclear site. The results are discussed in the context of experimental observations, which display a preference for mixed oxide systems.

Study of Hypochlorite Reduction Related to the Sodium Chlorate Process

“…The core electrons were modelled by an ultrasoft pseudopotential [30]. Following previous work a high spin electronic configuration in combination with a ferromagnetic coupling between the transition metal sites was assumed [31,32]. The electrochemistry was modelled using a binuclear Cr or Fe site embedded into a MgO y (OH) z test rig.…”

“…The validity of this simple yet efficient model system has been shown in a series of studies concerning electrochemical water splitting. It was found that the test rig system is able to reproduce trends with a semi-quantitative accuracy [31,[34][35][36].…”

From absolute potentials to a generalized computational standard hydrogen electrode for aqueous and non-aqueous solvents

“…21,22 This intrinsically connects the PCET step to the standard hydrogen electrode (SHE) scale. Naturally, also any other reference may be used, 15,23 provided its redox potential is known with sufficient accuracy.…”

Selectivity of Nanocrystalline IrO2-Based Catalysts in Parallel Chlorine and Oxygen Evolution

“…The formation of the oxygen molecule at the electrocatalyst surface can proceed through either a mononuclear [13,14,15,16] or a binuclear [17,18] pathway.…”